In the process of recovering (collecting) crude oil from oil reservoirs, different recovery processes are applied in time series, that is, a three-step method including primary, secondary, and tertiary (or EOR (enhanced)) processes is applied.
The primary recovery method includes: natural flowing using natural pressure of oil reservoirs and gravity; and artificial lifting using artificial recovery techniques such as pumps. The crude oil recovery ratio of the primary recovery carried out by these methods in combination is said to be about 20% at maximum. The secondary recovery method includes water flooding and pressure maintenance, which are intended to restore oil reservoir pressure and to increase oil production by injecting water or natural gas after the production decreases in the primary recovery method. With these primary and secondary recovery methods in combination, the crude oil recovery ratio is about 40%, and a large amount of crude oil remains in the underground oil reservoir. The tertiary recovery method is then proposed, which is a method of recovering crude oil through the EOR process, to recover more crude oil further from the oil reservoir in which crude oil has already been collected from an easy-to-recover section.
The EOR process includes thermal flood, gas flood, microbial EOR, and chemical flood. The chemical flood, including polymer flooding, surfactant flooding, micellar flooding, is a process for improving the crude oil recovery ratio by pressing a chemical fluid suited for the purpose into an oil reservoir to enhance flowability of crude oil, reducing surface tension acting between water and oil, or creating a micellar state between the pressed gas and oil.
Surfactant flooding is a process of pressing a series of fluids including a fluid mainly composed of a surfactant into an oil reservoir to reduce the interfacial tension between crude oil and water to extract and collect the trapped crude oil by capillarity. In this process, for example, alkyl aryl sulfonate is singly used as a surfactant, or alkyl aryl sulfonate is used in combination with an auxiliary surfactant and/or an auxiliary agent. Alkyl aryl sulfonate is generally used because it can not only reduce the interfacial tension between oil and water but also exhibit behaviors of a variety of phases when used in combination with various salt concentrations, as described later. More specifically, at a low salt concentration, alkyl aryl sulfonate remains in the aqueous phase, whereas at a high salt concentration, it tends to remain in the oil phase. At a middle salt concentration, it is known that a microemulsion is formed, so that a considerable amount of oil and saltwater are present in the microemulsion phase to exhibit high crude oil recovery capability.
Micellar flooding is an oil recovery process in which a microemulsion is produced from water and crude oil, and the microemulsion called a micelle solution is injected to underground reservoirs. Many surfactants are disclosed for producing a micelle solution, see U.S. Pat. No. 3,506,070 “Use of Water-External Micellar Dispersions in Oil Recovery”, issued Apr. 14, 1970 to Marathon Oil Corporation and U.S. Pat. No. 3,613,786, “Oil Recovery Method Using High Water Content Oil-External Micellar Dispersion, issued on Oct. 19, 1971 to Marathon Oil Company and U.S. Pat. No. 3,740,343 “:”High Water Content Oil-Dispersion Micellar Dispersions” issued Jun. 19, 1973 to Marathon Oil Company.
As a surfactant used in this process, a variety of anionic, nonionic, and cationic surfactants are disclosed, such as petroleum sulfonates, alkyl aryl sulfonates, alkanesulfonates, polyoxyethylene alkyl ether sulfates, polyoxyethylene alkyl ethers, polyoxyethylene alkyl phenyl ethers, polyhydric alcohol fatty acid esters, and alkyl trimethyl ammonium salts.
A micelle solution for use in the recovery of oil is disclosed, which includes internal olefin sulfonate having 10 to 30 carbon atoms and α-olefin sulfonate having 10 to 30 carbon atoms (see Japanese Examined Patent Application Publication No. 1-35157).
One of conventional polymer flooding techniques, micellar polymer flooding involving pressing of both micellar slug (a mixture of petroleum sulfonate, auxiliary agent, seawater, and oil) and polymer fails to achieve a stable crude oil recovery ratio. Then, a chemical for crude oil recovery is proposed, which includes: a nonionic surfactant including an amide compound such as a reaction product of a fatty acid and an alkanolamine, and an alkylene oxide adduct thereof; and a water-soluble polymer. It is disclosed that a high crude oil recovery ratio is stably obtained (see Japanese Examined Patent Application Publication No. 5-86989).
Furthermore, a certain alkylxylene sulfonate is proposed as a surfactant for EOR with a low interfacial tension (see Japanese Patent No. 5026264).
U.S. Published Patent Application No. 2010/0096139, “Method for Intervention Operations in Subsurface Hydrocarbon Formations”, assigned to assigned to Frac Tech Services, Ltd. and Illinois Institute of Technology. discloses a method of efficiently removing oil drops adhering to the rock surface by injecting nanoparticles mixed in a wetting agent in an aqueous or hydrocarbon carrier fluid into hydrocarbon reservoirs or wells to enhance disjoining pressure. The nanoparticles have a particle size in the range of 1 to 100 nm (for example, silicon dioxide). The nanoparticles are mixed in a wetting agent in an aqueous or hydrocarbon carrier fluid which includes an α-olefin sulfonate. This fluid comprising nanoparticles is then injected into a hydrocarbon reservoir or well. To even further develop this effect, stability of nanoparticles in the wetting agent is required. This requirement, however, needs heat resistance of the wetting agent comprising nanoparticles. Moreover, to fulfill the effect in subsea hydrocarbon reservoirs or wells, salt resistance of the wetting agent comprising nanoparticles is also required. A method for improving recovery of crude oil, gas, and water from a hydrocarbon reservoir or well is disclosed. In this method, nanoparticles in the range of 1 to 100 nm (for example, silicon dioxide) mixed in a wetting agent in an aqueous or hydrocarbon carrier fluid including an α-olefin sulfonate are injected into a hydrocarbon reservoir or well.”
On the other hand, an anode deposition-type electrodeposition coating material composition is disclosed, which includes: an acrylic polycarboxylic acid resin, for example, neutralized with amine or ammonium; a hardener; and a colloidal silica surface-treated with a silane coupling agent (see Japanese Patent No. 4033970).
In order to even further develop effective fluid treatments, stability of nanoparticles in the wetting agent is required. This requirement, however, needs heat resistance of the wetting agent comprising nanoparticles. Moreover, to fulfill the effect in subsea hydrocarbon reservoirs or wells, salt resistance of the wetting agent comprising nanoparticles is also required.
The presence of an anionic surfactant having an effect of removing crude oil adhering to sandstones or rocks such as carbonate rock or the like in subsurface or subsea oil reservoirs is essential for improving crude oil recoverability of a crude oil recovery chemical fluid. However, as the anionic surfactant has poor resistance to high temperature and salt, it is decomposed in a short time by injecting it into oil reservoirs having a high temperature and salt concentration, and thus it cannot exert fully crude oil recovery effect. In addition, although it is said that colloidal silica has crude oil recovery effect, the colloidal silica itself also has poor resistance to high temperature and salt, it becomes a gel in a short time by injecting it into oil reservoirs having a high temperature and salt concentration, and thus it cannot exert fully crude oil recovery effect.
Therefore, there has been a demand for crude oil recovery chemicals that can simultaneously achieve heat resistance and salt resistance and implement efficient crude oil recovery. Particularly, crude oil recovery chemical fluids are often collected several months after they are injected into the subsurface or subsea oil reservoirs. There has been a demand for the chemicals which can exhibit crude oil recovery effect and are stable even under unusual and severe environments being exposed to seawater or a saltwater comprising sodium ion, potassium ion and chlorine ion, and the like in a high concentration at a high temperature such as 100° C., over several months.